In recent years, with the widespread use of a mobile phone or an information terminal device (for example, a smartphone, a tablet, etc.), there is a greater opportunity to view a browser, an SNS (Social Networking Service), an electronic book, etc., and there has been a tendency that the operating time of the device increased.
A mobile phone or an information terminal device which has a power saving function to enable a long period of use has been known. A power saving function which turns off a light (backlight) of a display screen if there is no operation on the touch panel for a predetermined time, and thereafter, turns off the display if there is still no operation after the predetermined time has also been known.
In the portable terminal device described in Patent Document 1, if change in temperature of a housing is detected on the basis of a detection signal of a temperature sensor, the backlight is turned on, and if change in temperature in a direction opposite to the change in temperature is detected on the basis of the detection signal of the temperature sensor, the backlight is turned off, thereby achieving power saving.